U.S. patent application number 10/954330 was filed with the patent office on 2006-03-30 for optical films and process for making them.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Charles C. Anderson, Marcus S. Bermel, Jehuda Greener.
Application Number | 20060068128 10/954330 |
Document ID | / |
Family ID | 35478623 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060068128 |
Kind Code |
A1 |
Greener; Jehuda ; et
al. |
March 30, 2006 |
Optical films and process for making them
Abstract
A method of film fabrication is taught that uses a coating and
drying apparatus to fabricate resin films suitable for optical
applications. In particular, resin films are prepared by
simultaneous application of multiple liquid layers to a moving
carrier substrate having low surface energy. After solvent removal,
the resin films are peeled from the sacrificial carrier substrate.
Films prepared by the current invention exhibit good dimensional
stability and low out-of-plane retardation.
Inventors: |
Greener; Jehuda; (Rochester,
NY) ; Anderson; Charles C.; (Penfield, NY) ;
Bermel; Marcus S.; (Pittsford, NY) |
Correspondence
Address: |
Paul A. Leipold;Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
35478623 |
Appl. No.: |
10/954330 |
Filed: |
September 30, 2004 |
Current U.S.
Class: |
428/1.1 |
Current CPC
Class: |
B29K 2001/12 20130101;
B32B 27/08 20130101; B29C 41/26 20130101; G02B 1/14 20150115; B29K
2027/00 20130101; C08J 2301/12 20130101; B29K 2067/00 20130101;
B29K 2001/00 20130101; B29D 11/00788 20130101; B29L 2011/00
20130101; Y10T 428/10 20150115; B29K 2029/00 20130101; B05C 9/06
20130101; B29K 2995/0032 20130101; B29L 2009/001 20130101; G02B
5/208 20130101; B05C 5/007 20130101; B29C 41/32 20130101; C08J 5/18
20130101; C08J 2369/00 20130101; C09K 2323/00 20200801 |
Class at
Publication: |
428/001.1 |
International
Class: |
C09K 19/00 20060101
C09K019/00 |
Claims
1. A process for forming an optical resin film, having an
out-of-plane retardation (OPR) of less than 100 nm, comprising the
steps of: (a) applying a liquid optical resin /solvent mixture onto
the surface of a moving discontinuous carrier substrate having a
surface energy level less than 35 erg/cm.sup.2; (b) drying the
liquid resin/solvent mixture to substantially remove the solvent
yielding a composite of a resin film weakly adhered to the carrier
substrate, the resin film being releasably adhered to the carrier
substrate thereby allowing the resin film to be peeled from the
carrier substrate, and (c) removing the film from the substrate,
with the formed film exhibiting OPR of less than 100 nm, and in
plane retardation of less than 20 nm.
2. A process as recited in claim 1 wherein: the liquid
resin/solvent mixture is applied onto a discontinuous carrier
substrate having a length of 1 Om or more.
3. A process as recited in claim 1 wherein: the liquid
resin/solvent mixture is applied using a roll-to-roll process.
4. A process as recited in claim 1 wherein: the liquid
resin/solvent mixture is applied using slide bead coating die with
a multilayer composite being formed on a slide surface thereof.
5. A process as recited in claim 2 wherein: the viscosity of each
liquid layer of the multilayer composite is less than 5000 cp.
6. A process as recited in claim 1 wherein: the carrier substrate
is polyethylene terephthalate.
7. A process as recited in claim 1 wherein: the carrier substrate
has a surface layer applied to the coated surface and the surface
energy of said layer is less than 35 erg/cm.sup.2.
8. A process as recited in claim 2 wherein: an uppermost layer of
the multilayer composite contains a surfactant.
9. A process as recited in claim 2 wherein: at least a top layer of
the multilayer composite contains a polysiloxane surfactant.
10. A process as recited in claim 1 further comprising the step of:
winding the composite into at least one roll before the optical
resin film is peeled from the discontinuous carrier substrate.
11. A process as recited in claim 1 further comprising the steps
of: (a) separating the resin film from the carrier substrate
immediately after the drying step; and (b) winding the optical
resin film into at least one roll.
12. The process of claim 8 further comprising the step of:
unwinding at least a portion of at least one roll of the composite;
and separating the resin film from the carrier substrate.
13. The process of claim 1 wherein said optical resin comprises
cellulose ester and out of plane retardation is less than 20
nm.
14. A process as recited in claim 1 wherein: the resin film is
adhered to the carrier substrate with an adhesive strength of less
than about 250 N/m.
15. A process as recited in claim 11 further comprising the step
of: reducing residual solvent in the optical resin film to less
than 10% by weight prior to the separating step.
16. A process as recited in claim 12 further comprising the step
of: reducing residual solvent in the optical resin film to less
than 10% by weight prior to the separating step.
17. A process as recited in claim 2 wherein: at least a top layer
of the multilayer composite contains a fluorinated surfactant.
18. A process as recited in claim 1 wherein: the resin film has an
in-plane retardation of less than 10 nm and an OPR of less than 10
nm.
19. A process as recited in claim 1 wherein: the resin film has an
in-plane retardation of between 0.5 and 5 nm and an OPR of between
0.5 and 5 nm.
20. A process as recited in claim 1 further comprising the step of:
applying at least one additional resin layer to the composite after
the drying step.
21. A process as recited in claim 1 wherein: the resin film has a
thickness in the range of 1 to 100 .mu.m.
22. The process of claim 1 wherein said film comprises resin
selected from the group consisting of polycarbonates, polyesters,
cellulosics, polyolefins, acrylics, styrenics, polyamides and
polyester amides.
23. The process of claim 1 wherein said discontinuous carrier
substrate has a coating surface comprising a fluorinated
polymer.
24. The process of claim 1 wherein said discontinuous carrier
substrate has a coating surface comprising a polyolefin.
25. The process of claim 1 wherein said discontinuous carrier
substrate has a coating surface comprising a silicon-based
polymer.
26. The process of claim 1 wherein said optical resin comprises
triacetyl cellulose.
27. The process of claim 1 wherein said optical resin film includes
one or more UV absorbers.
28. A composite element comprising: a resin film coated on a
discontinuous carrier substrate, the resin film having a thickness
in the range of from 1 to 100 .mu.m, the resin film having an
in-plane retardation that is less than 20 nm and an OPR less than
20 nm, the resin film being adhered to the carrier substrate with
an adhesive strength of less than about 250 N/m.
29. A composite element as recited in claim 28 wherein: the resin
film has an in-plane retardation that is less than 10 nm and an OPR
less than 10 nm.
30. A composite element as recited in claim 28 wherein: the resin
film has an in-plane retardation that is between 0.5 and 5 nm and
an OPR of between 0.5 and 5 nm.
31. A composite element as recited in claim 28 wherein: the resin
film is adhered to the carrier substrate with an adhesive strength
of at least about 0.3 N/m.
32. A composite element as recited in claim 28 wherein: the resin
film is peelable from the carrier substrate.
33. A composite element as recited in claim 28 wherein: the resin
film is a multilayer composite.
34. A composite element as recited in claim 33 wherein: at least a
top layer of the multilayer composite includes a surfactant
therein.
35. A composite element as recited in claim 28 wherein: a
plasticizer is incorporated in the optical resin film.
36. A composite element as recited in claim 28 wherein: one or more
UV absorbers are incorporated in the optical resin film.
37. The composite element of claim 28 wherein the resin film
comprises cellulose ester.
38. A composite element comprising: a polycarbonate resin film at
least 10 meters in length coated on a discontinuous carrier
substrate, the resin film having a thickness in the range of from 1
to 100 .mu.m, the resin film having an in-plane retardation that is
less than 20 nm and an out of plane retardation less than 100 nm,
the resin film being adhered to the carrier substrate with an
adhesive strength of less than about 250 N/m.
39. A composite element as recited in claim 38 wherein: the resin
film has an in-plane retardation that is between 0.5 and 5 nm and
an out of plane retardation of less than 80 nm.
40. A composite element as recited in claim 38 wherein: the resin
film is adhered to the carrier substrate with an adhesive strength
of at least about 0.3 N/m.
41. A composite element as recited in claim 38 wherein: the resin
film is peelable from the carrier substrate.
42. A composite element as recited in claim 38 wherein: the resin
film is a multilayer composite.
43. A composite element as recited in claim 38 wherein: at least a
top layer of the multilayer composite includes a surfactant
therein.
44. A composite element as recited in claim 38 wherein: a
plasticizer is incorporated in the optical resin film.
45. A composite element as recited in claim 38 wherein: one or more
UV absorbers are incorporated into the optical resin film.
46. A resin film comprising: a layer of resin formed by a coating
operation, the resin film having a thickness in the range of from 1
to 100 .mu.m, the resin film having an in-plane retardation that is
less than 20 nm and an out-of-plane retardation of less than 20
nm.
47. A resin film as recited in claim 46 wherein: the optical resin
film having an in-plane retardation that is less than 10 nm and an
out-of-plane retardation less than 10 nm.
48. A resin film as recited in claim 46 wherein: the resin film
having an in-plane retardation that is between 0.5 and 5 nm and an
out-of-plane retardation of between 0.5 and 5 nm.
49. A liquid crystal display comprising a resin film comprising: a
layer of resin formed by a coating operation, the resin film having
a thickness in the range of from 1 to 100 .mu.m, the resin film
having an in-plane retardation that is less than 20 nm and an
out-of-plane retardation of less than 20 nm.
50. A liquid crystal display comprising as recited in claim 49
wherein: the resin film has an in-plane retardation that is less
than 10 nm and an OPR less than 10 nm.
51. A liquid crystal display comprising as recited in claim 49
wherein: the resin film has an in-plane retardation that is between
0.5 and 5 nm and an OPR of between 0.5 and 5 nm.
52. A composite film as recited in claim 49 wherein: at least a top
layer of the multilayer composite includes a fluorinated surfactant
therein.
53. A liquid crystal display comprising a resin film comprising: a
layer of polycarbonate resin formed by a coating operation, the
resin film having a thickness in the range of from 5 to 100 .mu.m,
the resin film having an in-plane retardation that is less than 20
nm and an out-of-plane retardation of less than 100 nm.
54. A liquid crystal display comprising as recited in claim 53
wherein: the resin film has an in-plane retardation that is less
than 10 nm and an OPR less than 80 nm.
55. A composite film as recited in claim 54 wherein: at least a top
layer of the multilayer composite includes a fluorinated surfactant
therein.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to methods for
manufacturing resin films and, more particularly, to an improved
method for the manufacture of optical films used to form electrode
substrates, light polarizers, compensation plates, and protective
covers in optical devices such as liquid crystal displays and other
electronic displays where the films exhibit good dimensional
stability and both low in-plane and low out-of-plane
retardation.
BACKGROUND OF THE INVENTION
[0002] Transparent resin films are used in a variety of optical
applications. In particular, resin films are used as protective
cover sheets for light polarizers, compensation films and as
electrode substrates in variety of electronic displays. In this
regard, optical films are intended to replace glass to produce
lightweight, flexible display screens. These display screens
include liquid crystal displays, OLED (organic light emitting
diode) displays, and other electronic displays found in, for
example, personal computers, televisions, cell phones, and
instrument panels.
[0003] Electronic display screens such as liquid crystal displays
(LCD's) contain a number of optical elements that may be formed
from resin films. The structure of reflective LCD's may include a
liquid crystal cell, one or more polarizer plates, and one or more
compensation films. Liquid crystals cells are formed by dispersing
twisted nematic (TN) or super twisted nematic (STN) materials
between two electrode substrates. For liquid crystals cells, resin
substrates have been suggested as lightweight and flexible
alternatives to glass substrates. Like glass, resin electrode
substrates must be transparent, exhibit very low birefringence, and
withstand the high temperatures required to vapor deposit a
transparent conductive material, such as indium-tin oxide, onto the
surface of the film. Suitable thermally stable resins suggested for
electrode substrates include polycarbonates, sulfones, cyclic
olefins, and polyarylates.
[0004] Polarizer plates are typically a multi-layer element of
resin films and are comprised of a polarizing film sandwiched
between two protective cover sheets. Polarizing films are normally
prepared from a transparent and highly uniform amorphous resin film
that is subsequently stretched to orient the polymer molecules and
stained with a dye to produce a dichroic film. An example of a
suitable resin for the formation of polarizer films is fully
hydrolyzed polyvinyl alcohol (PVA). Because the stretched PVA films
used to form polarizers are very fragile and dimensionally
unstable, protective cover sheets are normally laminated to both
sides of the PVA film to offer both support and abrasion
resistance. Protective cover sheets of polarizer plates are
required to have high uniformity, good dimensional and chemical
stability, and high transparency. Originally, protective
coversheets were formed from glass, but a number of resin films are
now used to produce lightweight and flexible polarizers. Although
many resins have been suggested for use in protective cover sheets
including, cellulosics, acrylics, cyclic olefin polymers,
polycarbonates, and sulfones, acetyl cellulose polymers are most
commonly used in protective cover sheets for polarizer plates.
Polymers of the acetyl cellulose type are commercially available in
a variety of molecular weights as well as the degree of acyl
substitution of the hyroxyl groups on the cellulose backbone. Of
these, the fully substituted polymer, triacetyl cellulose (TAC) is
commonly used to manufacture resin films for use in protective
cover sheets for polarizer plates.
[0005] The cover sheet normally requires a surface treatment to
insure good adhesion to the PVA dichroic film. When TAC is used as
the protective cover film of a polarizer plate, the TAC film is
subjected to treatment in an alkali bath to saponify the TAC
surface to provide suitable adhesion to the PVA dichoroic film. The
alkali treatment uses an aqueous solution containing a hydroxide of
an alkali metal, such as sodium hydroxide or potassium hydroxide.
After alkali treatment, the cellulose acetate film is typically
washed with weak acid solution followed by rinsing with water and
drying. This saponification process is both messy and time
consuming. U.S. Pat. No. 2,362,580 describes a laminar structure
wherein two cellulose ester films each having a surface layer
containing cellulose nitrate and a modified PVA is adhered to both
sides of a PVA film. JP 06094915A discloses a protective film for
polarizer plates wherein the protective film has a hydrophilic
layer which provides adhesion to PVA film.
[0006] Some LCD devices may contain a polarizer plate having a
protective cover sheet that also serves as a compensation film to
improve the viewing angle of an image. Alternatively, the LCD
device may contain one or more separate films that serve as the
compensation films. Compensation films (i.e. retardation films or
phase difference films) are normally prepared from amorphous films
that have a controlled level of birefringence produced either by
uniaxial stretching of the film or by coating the film with an
optically anisotropic layer. Suitable resins suggested for
formation of compensation films by stretching include polyvinyl
alcohols, polycarbonates and sulfones. Compensation films prepared
by coating with an anisotropic layer normally require highly
transparent resin films having low birefringence such as TAC and
cyclic olefin polymers.
[0007] Protective cover sheets may require the application of other
functional layers (herein also referred to as auxiliary layers)
such as an antiglare layer, antireflection layer, anti-smudge
layer, or antistatic layer. Generally, these functional layers are
applied in a process step that is separate from the manufacture of
the resin film.
[0008] Regardless of their end usage, the precursor resin films
used to prepare the various types of optical components described
above are generally desired to have high transparency, high
uniformity, and low birefringence. Moreover, these films may be
needed in a range of thickness depending on the final
application.
[0009] In general, resin films are prepared either by melt
extrusion methods or by casting methods. Melt extrusion methods
involve heating the resin until molten (approximate viscosity on
the order of 100,000 cp), and then applying the hot molten polymer
to a highly polished metal band or drum with an extrusion die,
cooling the film, and finally peeling the film from the metal
support. For many reasons, however, films prepared by melt
extrusion are generally not suitable for optical applications.
Principal among these is the fact that melt extruded films exhibit
a high degree of optical birefringence. In the case of many
polymers there is the additional problem of melting the polymer.
For example, highly saponified polyvinyl alcohol has a very high
melting temperature of 230 degrees Celsius, and this is above the
temperature where discoloration or decomposition begins (.about.200
degrees Celsius). Similarly, cellulose triacetate polymer has a
very high melting temperature of 270-300.degree. C., and this is
above the temperature where decomposition begins. In addition, melt
extruded films are known to suffer from other artifacts such as
poor flatness, pinholes and inclusions. Such imperfections may
compromise the optical and mechanical properties of optical films.
For these reasons, melt extrusion methods are generally not
suitable for fabricating many resin films intended for optical
applications. Rather, casting methods are generally used to
producing these films.
[0010] Resin films for optical applications are manufactured almost
exclusively by casting methods. Casting methods involve first
dissolving the polymer in an appropriate solvent to form a dope
having a high viscosity on the order of 50,000 cp, and then
applying the viscous dope to a continuous highly polished metal
band or drum through an extrusion die, partially drying the wet
film, peeling the partially dried film from the metal support, and
conveying the partially dried film through an oven to more
completely remove solvent from the film. Cast films typically have
a final dry thickness in the range of 40-200 .mu.m. In general,
thin films of less than 40 .mu.m are very difficult to produce by
casting methods due to the fragility of wet film during the peeling
and drying processes. Films having a thickness of greater than 200
.mu.m are also problematic to manufacture due to difficulties
associated with the removal of solvent in the final drying step.
Although the dissolution and drying steps of the casting method add
complexity and expense, cast films generally have better optical
properties when compared to films prepared by melt extrusion
methods, and problems associated with high temperature processing
are avoided.
[0011] Examples of optical films prepared by casting methods
include: 1.) Polyvinyl alcohol sheets used to prepare light
polarizers as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S.
Pat. No. 5,925,289 to Cael as well as more recent disclosures in
U.S. patent application Ser. No. 2001/0039319 A1 to Harita and U.S.
patent application Ser. No. 2002/001700 A1 to Sanefuji, 2.)
Cellulose triacetate sheets used for protective covers for light
polarizers as disclosed in U.S. Pat. No. 5,695,694 to Iwata, 3.)
Polycarbonate sheets used for protective covers for light
polarizers or for retardation plates as disclosed in U.S. Pat. No.
5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180
both to Taketani, and 4.) Polysulfone sheets used for protective
covers for light polarizers or for retardation plates as disclosed
in U.S. Pat. Nos. 5,611,985 to Kobayashi and U.S. Pat. Nos.
5,759,449 and 5,958,305 both to Shiro.
[0012] One disadvantage to the casting method is that cast films
have significant optical birefringence. Although films prepared by
casting methods have lower birefringence when compared to films
prepared by melt extrusion methods, birefringence remains
objectionably high. For example, cellulose triacetate films
prepared by casting methods exhibit in-plane retardation of 7
nanometers (nm) for light in the visible spectrum as disclosed in
U.S. Pat. No. 5,695,694 to Iwata. A polycarbonate film prepared by
the casting method is disclosed as having an in-plane retardation
of 17 nm in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to
Taketani. U.S. patent application Ser. No. 2001/0039319 A1 to
Harita claims that color irregularities in stretched polyvinyl
alcohol sheets are reduced when the difference in retardation
between widthwise positions within the film is less than 5 nm in
the original unstretched film.
[0013] Commonly-assigned U.S. patent application Publications Nos.
2003/0215658A, 2003/0215621A, 2003/0215608A, 2003/0215583A,
2003/0215582A, 2003/0215581A, 2003/0214715A to Bermel describe a
coating method to prepare resin films having low in-plane
retardation that are suitable for optical applications. Bermel is
silent with respect to the importance of out-of-plane retardation
or the means to achieve low out-of-plane retardation values. In
these Bermel references the resin films are applied onto a
discontinuous, sacrificial substrate from lower viscosity polymer
solutions than are normally used to prepare cast films. A wide
variety of substrates are disclosed including those having high
surface energies such as untreated polyethylene terephthalate
(PET), glass, and aluminum (suface energy values of 47, 49.4, and
49 erg/cm.sup.2, respectively).
[0014] For some applications of optical films, both low in-plane
and low out-of-plane retardation values are desirable. In
particular, values of in-plane retardation and out-of-plane
redardation of less than 10 nm may be preferred.
[0015] Birefringence in cast films arises from orientation of
polymers during the manufacturing operations. This molecular
orientation causes indices of refraction within the plane of the
film to be measurably different. Two components of birefringence
are usually considered in the characterization of optical films,
both of which can impact in different ways the performance of
optical devices comprising said films. In-plane birefringence is
the difference between indices of refraction for polarized light
traversing in perpendicular directions normal to the film plane.
The out-of-plane birefringence represents the difference between
the average of two refractive indices for light traversing in two
perpendicular directions normal to the film plane and the
refractive index for light traversing parallel to the film surface.
The absolute value of birefringence multiplied by the film
thickness is defined as in-plane retardation. Therefore, in-plane
retardation and out-of-plane retardation are two independent
measures of the molecular anisotropy within the plane of the
film.
[0016] During the casting process, molecular orientation may arise
from a number of sources, including shear of the dope in the die,
shear of the dope by the metal support during application, shear of
the partially dried film during the peeling step, and shear of the
free-standing film during conveyance through the final drying step.
These shear forces orient the polymer molecules and ultimately give
rise to undesirably high birefringence or retardation values. To
minimize shear and obtain the lowest birefringence films, casting
processes are typically operated at very low line speeds of 1-15
m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line
speeds generally produce the highest quality films. This approach
can minimize the in-plane retardation, however the out-of-plane
retardation may still be quite high. The out-of-plane retardation
is often caused by drying stresses generated in the vicinity of an
adhering surface. EPA 0 380 02B to Machell and Greener disclosed
that by lowering the adhesion of the casting solution to the
substrate in a batch casting method, the out-of-plane retardation
of the film can be reduced. This can be accomplished, e.g., by
lowering the surface energy of the substrate.
[0017] Another drawback to the casting method is the inability to
accurately apply multiple layers. As noted in U.S. Pat. No.
5,256,357 to Hayward, conventional multi-slot casting dies create
unacceptably non-uniform films. In particular, line and streak
non-uniformity is greater than 5% with prior art devices.
Acceptable two layer films may be prepared by employing special die
lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but
the die designs are complex and may be impractical for applying
more than two layers simultaneously.
[0018] Another drawback to the casting method is the restrictions
on the viscosity of the dope. In casting practice, the viscosity of
dope is on the order of 50,000 cp. For example, U.S. Pat. No.
5,256,357 to Hayward describes practical casting examples using
dopes with a viscosity of 100,000 cp. In general, cast films
prepared with lower viscosity dopes are known to produce
non-uniform films as noted for example in U.S. Pat. No. 5,695,694
to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity
dopes used to prepare casting samples are approximately 10,000 cp.
At these high viscosity values, however, casting dopes are
difficult to filter and degas. While fibers and larger debris may
be removed, softer materials such as polymer slugs are more
difficult to filter at the high pressures found in dope delivery
systems. Particulate and bubble artifacts create conspicuous
inclusion defects as well as streaks and may create substantial
waste.
[0019] In addition, the casting method can be relatively inflexible
with respect to product changes. Because casting requires high
viscosity dopes, changing product formulations requires extensive
down time for cleaning delivery systems to eliminate the
possibility of contamination. Particularly problematic are
formulation changes involving incompatible polymers and solvents.
In fact, formulation changes are so time consuming and expensive
with the casting method that most production machines are dedicated
exclusively to producing only one film type.
[0020] The manufacture of resin films by the casting method is also
confounded by a number of artifacts associated with the stripping
and conveyance operations. Stripping operations, for example,
frequently require converting aids such as special co-solvents or
additives in the casting formulation to facilitate peeling of the
film from the metal substrate without creating streak artifacts. In
fact, stripping can be so problematic that some films such as
polymethylmethacrylate films can not be manufactured by casting
methods without resorting to specialty co-polymers as noted in U.S.
Pat. Nos. 4,584,231 and 4,664,859 both to Knoop. In addition to
stripping artifacts, cast films may be damaged during conveyance
across numerous rollers during the final drying operation. For
example, abrasion, scratch and wrinkle artifacts have be noted in
polycarbonate films as described in U.S. Pat. No. 6,222,003B1 to
Hosoi. To minimize damage during conveyance, cast polycarbonate
films require special additives that act as lubricants or surface
modifiers, or require a protective laminate sheet, or require
knurled edges. However, special additives may compromise film
clarity. Moreover, lamination and edge knurling devices are
expensive and add complexity to the casting process.
[0021] Finally, cast films may exhibit undesirable cockle or
wrinkles. Thinner films are especially vulnerable to dimensional
artifacts either during the peeling and drying steps of the casting
process or during subsequent handling of the film. In particular,
the preparation of composite optical plates from resin films
requires a lamination process involving application of adhesives,
pressure, and high temperatures. Very thin films are difficult to
handle during this lamination process without wrinkling. In
addition, many cast films may naturally become distorted over time
due to the effects of moisture. For optical films, good dimensional
stability is necessary during storage as well as during subsequent
fabrication of composite optical plates.
[0022] It is a problem to be solved to provide an optical resin
film composite comprising a polymer film that exhibit good
dimensional stability and both low in-plane birefringence and low
out-of-plane birefringence and a process for forming such
films.
SUMMARY OF THE INVENTION
[0023] The invention provides a process for forming an optical
resin film, having an out-of-plane retardation of less than 100 nm
and an in-plane retardation of less than 20 nm, comprising the
steps of:
[0024] (a) applying a liquid optical resin /solvent mixture onto
the surface of a moving discontinuous carrier substrate in a
roll-to-roll process having a surface energy level less than 35
erg/cm.sup.2;
[0025] (b) drying the liquid resin/solvent mixture to substantially
remove the solvent yielding a composite of a resin film weakly
adhered to the carrier substrate, the resin film being releasably
adhered to the carrier substrate thereby allowing the resin film to
be peeled from the carrier substrate, and
[0026] (c) removing the film from the substrate, with the formed
film exhibiting out-of-plane of less than 100 nm and in-plane
retardation of less than 20 nm.
[0027] The invention also provides an optical resin film, a
composite element, a polarizer plate and a display device. Optical
resin films prepared by the current invention exhibit good
dimensional stability and low in-plane and out-of-plane
birefringence.
[0028] The fabrication of these optical resin films is facilitated
by the carrier substrate that supports the wet optical film coating
through the drying process and eliminates the need to peel the film
from a metal band or drum prior to a final drying step as required
in the casting methods described in prior art. Rather, the optical
film is completely dried before separation from the carrier
substrate. In fact, the composite element comprising the optical
film and carrier substrate are preferably wound into rolls and
stored until needed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic of an exemplary roll-to-roll coating
and drying apparatus that can be used in the practice of the method
of the present invention.
[0030] FIG. 2 is a schematic of an exemplary roll-to-roll coating
and drying apparatus of FIG. 1 including a station where the resin
web separated from the substrate is separately wound.
[0031] FIG. 3 is a schematic of an exemplary multi-slot coating
apparatus that can be used in the practice of the method of the
present invention.
[0032] FIG. 4 shows a cross-sectional representation of a resin
film formed by the method of the present invention that is
partially peeled from a carrier substrate having low surface
energy.
[0033] FIG. 5 shows a cross-sectional representation of a resin
film formed by the method of the present invention that is
partially peeled from a carrier substrate having a low surface
energy subbing layer formed thereon.
[0034] FIG. 6 shows a cross-sectional representation of a
multi-layer resin film formed by the method of the present
invention that is partially peeled from a carrier substrate having
low surface energy.
[0035] FIG. 7 shows a cross-sectional representation of a
multi-layer resin film formed by the method of the present
invention that is partially peeled from a carrier substrate having
a low surface energy subbing layer formed thereon.
[0036] FIG. 8 is a schematic of a casting apparatus as used in
prior art to cast resin films.
[0037] FIG. 9 shows the dependence of the out-of-plane retardation
on film thickness for a solid polycarbonate resin cast from a
methylene chloride solution onto a variety of substrates with
different surface energies.
DETAILED DESCRIPTION OF THE INVENTION
[0038] It is an object of the present invention to overcome the
limitations of prior art casting methods and provide a new
roll-to-roll coating method for preparing amorphous polymeric films
having very low levels of both in-plane and out-of-plane
birefringence. It is a further object of the present invention to
provide a new method of producing highly uniform polymeric films
over a broad range of dry thicknesses.
[0039] Yet another object of the present invention is to provide a
method of preparing polymeric films by simultaneously applying
multiple layers to a moving substrate in a roll-to-roll process.
Still another object of the present invention is to provide a new
method of preparing polymeric films with improved dimensional
stability and handling ability by temporarily adhering the resin
film to a supporting carrier substrate at least until it is
substantially dry and then subsequently separating the carrier
substrate from the resin film.
[0040] A further object of the present invention is to provide a
composite element comprising a resin film coated on a discontinuous
carrier substrate, the substrate having a low surface energy and
the resin film having an in-plane retardation that is less than 20
nm and an out-of-plane retardation that is less than 100 nm, the
resin film being adhered to the carrier substrate with an adhesive
strength of less than about 250 N/m. Another object of the present
invention is to provide a resin film comprising a layer of a
polycarbonate formed by a coating operation, the polycarbonate film
having an in-plane retardation that is less than 20 nm and an
out-of-plane retardation that is less than 100 nm. A still further
object of the present invention is to provide a resin film
comprising a layer of a cellulose ester formed by a coating
operation, the cellulose ester film having an in-plane retardation
that is less than 20 nm and an out-of-plane retardation that is
less than 20 nm.
[0041] Briefly stated, the foregoing and numerous other features,
objects and advantages of the present invention will become readily
apparent upon review of the detailed description, claims and
drawings set forth herein. These features, objects and advantages
are accomplished by applying a low viscosity fluid containing
polymeric resins onto a moving, discontinuous carrier substrate by
a coating method. Unlike the continuous metal bands or wheels
typically used to cast resin films, the discontinuous carrier
substrate employed in the present invention is a noncontinuous
substrate having length of at least 10 meters, preferably at least
1000 meters. The carrier substrate is modified to possess surface
energy less than 35 erg/cm.sup.2. Preferably, the surface energy is
between 15 and 35 erg/cm.sup.2. Above a surface energy of 35
erg/cm.sup.2 it is very difficult to achieve low out-of-plane
retardation films. Surface energies below about 15 erg/cm.sup.2 are
impractical from the standpoint of coating, wetting and providing
sufficient adhesion of the dried resin film to the substrate. The
resin film is not separated from the carrier substrate until the
coated film is substantially dry (<10% residual solvent by
weight). In fact, the composite structure of resin film and carrier
substrate may be wound into rolls and stored until needed.
Typically, these rolls are at least 10 meters in length, preferably
the rolls are 1000 meters or more in length. Thus, the carrier
substrate cradles the optical resin film and protects against
shearing forces during conveyance through the drying process.
Moreover, because the resin film is dry and solid when it is
finally peeled from the carrier substrate, there is no shear or
orientation of polymer within the film due to the peeling process.
As a result, films prepared by the current invention exhibit very
low levels of in-plane and out-of-plane birefringence.
[0042] Polymeric films can be made with the method of the present
invention having a thickness of about 1 to 200 microns. Very thin
resin films of less than 40 microns can be easily manufactured at
line speeds not possible with prior art methods. The fabrication of
very thin films is facilitated by a carrier substrate that supports
the wet film through the drying process and eliminates the need to
peel the film from a metal band or drum prior to a final drying
step as required in the casting methods described in prior art.
Rather, the film is substantially if not completely dried before
separation from the carrier substrate. In all cases, dried resin
films have a residual solvent content of less than 10% by weight.
In a preferred embodiment of the present invention, the residual
solvent content is less than 5%, and most preferably less than 1%.
Thus, the present invention readily allows for preparation of very
delicate thin films not possible with the prior art casting method.
In addition, thick films of greater than 40 microns may also be
prepared by the method of the present invention. To fabricate
thicker films, additional coatings may be applied over a
film-substrate composite either in a tandem operation or in an
offline process without comprising optical quality. In this way,
the method of the present invention overcomes the limitation of
solvent removal during the preparation of thicker films since the
first applied film is dry before application of a subsequent wet
film. Thus, the present invention allows for a broader range of
final film thickness than is possible with casting methods.
[0043] In the method of the present invention, resin films are
created by forming a single layer or, preferably, a multilayer
composite on a slide surface of a coating hopper, the multilayer
composite including a bottom layer of low viscosity, one or more
intermediate layers, and an optional top layer containing a
surfactant, flowing the multilayer composite down the slide surface
and over a coating lip of the coating hopper, and applying the
multilayer composite to a moving substrate. In particular, the use
of the method of the present invention is shown to allow for
application of several liquid layers having unique composition.
Coating aids and additives may be placed in specific layers to
improve film performance or improve manufacturing robustness. For
example, multilayer application allows a surfactant to be placed in
the top spreading layer where needed rather than through out the
entire wet film. In another example, the concentration of polymer
in the lowermost layer may be adjusted to achieve low viscosity and
facilitate high-speed application of the multilayer composite onto
the carrier substrate. Therefore, the present invention provides an
advantageous method for the fabrication of multiple layer composite
films such as required for certain optical elements or other
similar elements.
[0044] Wrinkling and cockle artifacts are minimized with the method
of the present invention through the use of the carrier substrate.
By providing a stiff backing for the resin film, the carrier
substrate minimizes dimensional distortion of the optical film.
This is particularly advantageous for handling and processing very
thin films of less than about 40 microns. Moreover, scratches and
abrasion artifacts that are known to be created by the casting
method are avoided with the method of the present invention since
the carrier substrate lies between the resin film and potentially
abrasive conveyance rollers during all drying operations. In
addition, the restraining nature of the carrier substrate also
eliminates the tendency of resin films to distort or cockle over
time as a result of changes in moisture levels. Thus, the method of
the current invention insures that polymeric optical films are
dimensionally stable during preparation and storage as well as
during final handling steps necessary for fabrication of optical
elements.
[0045] In the practice of the method of the present invention it is
preferred that the substrate be a discontinuous sheet such as
polyethylene terephthalate (PET) that is conveniently supplied by
unwinding a roll of substrate having a length of at least 10
meters. The PET carrier substrate may be pretreated with a coated
surface layer or an electrical discharge device to modify its
surface energy. In particular, a coated surface layer may be
applied to lower the surface energy of the substrate, and allow the
film to be subsequently peeled away from the substrate while
producing low levels of out-of-plane retardation.
[0046] Although the present invention is discussed herein with
particular reference to a slide bead coating operation, those
skilled in the art will understand that the present invention can
be advantageously practiced with other coating operations. For
example, freestanding films having low in-plane and out-of-plane
retardation should be achievable with single or multiple layer slot
die coating operations and single or multiple layer curtain coating
operations. Moreover, those skilled in the art will recognize that
the present invention can be advantageously practiced with
alternative carrier substrates. For example, peeling films having
low in-plane and out-of-plane birefringence should be achievable
with other polymeric supports [e.g. polyethylene naphthalate (PEN),
cellulose acetate, PET], paper supports, resin laminated paper
supports, and metal supports (e.g. aluminum) so long as these
supports are treated to possess low surface energy.
[0047] Practical applications of the present invention include the
preparation of polymeric films used as optical films, laminate
films, release films, photographic films, and packaging films among
others. In particular, resin films prepared by the method of the
present invention may be utilized as optical elements in the
manufacture of electronic displays such as liquid crystal displays.
For example, liquid crystal displays are comprised of a number of
film elements including polarizer plates, compensation plates and
electrode substrates. Polarizer plates are typically a multilayer
composite structure having dichroic film (normally stretched
polyvinyl alcohol treated with iodine) with each surface adhered to
a protective cover having very low in-plane birefringence. The low
resin films prepared by the method of the present invention are
suitable protective sheets and also as precursor films for the
formation of light polarizers. The resin films prepared by the
method of the present invention are also suitable for the
manufacture of compensation plates and electrode substrates.
[0048] The films produced with the method of the present invention
are particularly useful for optical films. As produced, the films
made with the method of the present invention will have a light
transmittance of at least about 85 percent, preferably at least
about 90 percent, and most preferably, at least about 95 percent.
Further, as produced the films will have a haze value of less than
1.0 percent. In addition, the films are smooth with a surface
roughness average (Ra, ANSI Standard B46.1, 1985) of less than 100
nm and most preferrably with a surface roughness of less than 50
nm.
[0049] The terms "optical resins" and "optical films" as used
herein are used to describe any polymeric material that forms a
high clarity film with high light transmittance (i.e. >85%) and
low haze values (i.e.<1.0%). Examplary optical resins include
those described here, i.e. cellulose triacetate (also referred to
as triacetyl cellulose, TAC), polyvinyl alcohol, polycarbonate,
polyethersulfone, polymethylmethacrylate, and polyvinylbutyral.
Other potential optical resins might include fluoropolymers
(polyvinylidene fluoride, polyvinyl fluoride, and
polycholorotrifluorethene), other cellulosic esters (cellulose
diacetate, cellulose acetate butyrate, and cellulose acetate
propionate, for example), polyoefins (cyclic olefin polymers),
polystyrene, aromatic polyesters (polyarylates and polyethylene
terephthalate), sulfones (polysulfones, polyethersulfones,
polyarylsulfone), and polycarbonate copolymers, among others.
[0050] In one preferred embodiment, the optical resin film is a
polycarbonate film having an in-plane retardation that is less than
20 nm and an out-of-plane retardation that is less than 100 nm.
Most preferably the out-of-plane retardation is less than 80 nm.
Polycarbonates are condensation polymers having the general
structure of ##STR1## where R is an organic moiety derived from a
monomeric diol. Most common polycarbonates are derived from bis
phenol A monomer [2,2 bis (4 hydroxy-phenyl)propane] but other
monomers can be used in themselves or in combination with other
diols to form a multitude of polycarbonate structures. Most
polycarbonates, having an aromatic backbone structure, are
inherently birefringent materials that produce high levels of
out-of-plane retardation, typically much higher than that produced
by TAC resin, in films cast therefrom.
[0051] In another preferred embodiment, the optical resin film is a
cellulose ester film having an in-plane retardation that is less
than 20 nm and an out-of-plane retardation that is less than 20 nm.
Preferably the out-of-plane retardation is less than 10 nm, most
preferably less than 5 nm.
[0052] Turning first to FIG. 1 there is shown a schematic of an
exemplary and well known roll-to-roll coating and drying system 10
suitable for practicing the method of the present invention. The
coating and drying system 10 is typically used to apply very thin
films to a moving substrate 12 and to subsequently remove solvent
in a dryer 14. A single coating apparatus 16 is shown such that
system 10 has only one coating application point and only one dryer
14, but two or three (even as many as six) additional coating
application points with corresponding drying sections are known in
the fabrication of composite thin films. The process of sequential
application and drying is known in the art as a tandem coating
operation.
[0053] Coating and drying apparatus 10 includes an unwinding
station 18 to feed the moving substrate 12 around a back-up roller
20 where the coating is applied by coating apparatus 16. The coated
web 22 then proceeds through the dryer 14. In the practice of the
method of the present invention the final dry film 24 comprising a
resin film on substrate 12 is wound into rolls at a wind-up station
26.
[0054] As depicted, an exemplary four-layer coating is applied to
moving web 12. Coating liquid for each layer is held in respective
coating supply vessel 28, 30, 32, 34. The coating liquid is
delivered by pumps 36, 38, 40, 42 from the coating supply vessels
to the coating apparatus 16 conduits 44, 46, 48, 50, respectively.
In addition, coating and drying system 10 may also include
electrical discharge devices, such as corona or glow discharge
device 52, or polar charge assist device 54, to modify the
substrate 12 prior to application of the coating.
[0055] Turning next to FIG. 2 there is shown a schematic of the
same exemplary coating and drying system 10 depicted in FIG. 1 with
an alternative winding operation. Accordingly, the drawings are
numbered identically up to the winding operation. In the practice
of the method of the present invention, the dry film 24 comprising
a substrate (which may be a resin film, paper, resin coated paper
or metal) with a resin coating applied thereto is taken between
opposing rollers 56, 58. The resin film 60 is peeled from substrate
12 with the optical film going to winding station 62 and the
substrate 12 going to winding station 64. In a preferred embodiment
of the present invention, polyethylene terephthalate (PET) is used
as the substrate 12. The substrate 12 may be pretreated with a
subbing layer to modify the surface energy of the substrate 12.
[0056] The coating apparatus 16 used to deliver coating fluids to
the moving substrate 12 may be a multilayer applicator such as a
slide bead hopper, as taught for example in U.S. Pat. No. 2,761,791
to Russell, or a slide curtain hopper, as taught by U.S. Pat. No.
3,508,947 to Hughes. Alternatively, the coating apparatus 16 may be
a single layer applicator, such as a slot die hopper or a jet
hopper. In a preferred embodiment of the present invention, the
application device 16 is a multilayer slide bead hopper.
[0057] As mentioned above, coating and drying system 10 includes a
dryer 14 that will typically be a drying oven to remove solvent
from the coated film. An exemplary dryer 14 used in the practice of
the method of the present invention includes a first drying section
66 followed by eight additional drying sections 68-82 capable of
independent control of temperature and air flow. Although dryer 14
is shown as having nine independent drying sections, drying ovens
with fewer compartments are well known and may be used to practice
the method of the present invention. In a preferred embodiment of
the present invention the dryer 14 has at least two independent
drying zones or sections.
[0058] Preferably, each of drying sections 68-82 has independent
temperature and airflow controls. In each section, temperature may
be adjusted between 5.degree. C. and 150.degree. C. To minimize
drying defects from case hardening or skinning-over of the wet
film, optimum drying rates are needed in the early sections of
dryer 14. There are a number of artifacts created when temperatures
in the early drying zones are inappropriate. For example, fogging
or blush of polycarbonate films is observed when the temperature in
zones 66, 68 and 70 are set at 25.degree. C. This blush defect is
particularly problematic when high vapor pressure solvents
(methylene chloride and acetone) are used in the coating fluids.
Aggressively high temperatures are also associated with other
artifacts such as case hardening, reticulation patterns and
microvoids in the resin film. In a preferred embodiment of the
present invention, the first drying section 66 is operated at a
temperature of at least about 25.degree. C. but less than
95.degree. C. with no direct air impingement on the wet coating of
the coated web 22. In another preferred embodiment of the method of
the present invention, drying sections 68 and 70 are also operated
at a temperature of at least about 25.degree. C. but less than
95.degree. C. The actual drying temperature in drying sections 66,
68 may be optimized empirically within this range by those skilled
in the art.
[0059] Referring now to FIG. 3, a schematic of an exemplary coating
apparatus 16 is shown in detail. Coating apparatus 16,
schematically shown in side elevational cross-section, includes a
front section 92, a second section 94, a third section 96, a fourth
section 98, and a back plate 100. There is an inlet 102 into second
section 94 for supplying coating liquid to first metering slot 104
via pump 106 to thereby form a lowermost layer 108. There is an
inlet 110 into third section 96 for supplying coating liquid to
second metering slot 112 via pump 114 to form layer 116. There is
an inlet 118 into fourth section 98 for supplying coating liquid to
metering slot 120 via pump 122 to form layer 124. There is an inlet
126 into back plate 100 for supplying coating liquid to metering
slot 128 via pump 130 to form layer 132. Each slot 104, 112, 120,
128 includes a transverse distribution cavity. Front section 92
includes an inclined slide surface 134, and a coating lip 136.
There is a second inclined slide surface 138 at the top of second
section 94. There is a third inclined slide surface 140 at the top
of third section 96. There is a fourth inclined slide surface 142
at the top of fourth section 98. Back plate 100 extends above
inclined slide surface 142 to form a back land surface 144.
Residing adjacent the coating apparatus or hopper 16 is a coating
backing roller 20 about which a web 12 is conveyed. Coating layers
108, 116, 124, 132 form a multilayer composite which forms a
coating bead 146 between lip 136 and substrate 12. Typically, the
coating hopper 16 is movable from a non-coating position toward the
coating backing roller 20 and into a coating position. Although
coating apparatus 16 is shown as having four metering slots,
coating dies having a larger number of metering slots (as many as
nine or more) are well known and may be used to practice the method
of the present invention.
[0060] Coating fluids are comprised principally of polymeric resins
dissolved in a suitable solvent. Suitable resins include any
polymeric material that may be used to form a transparent film.
Practical examples of resins currently used to form optical films
include polyvinyl alcohols for polarizers, polyvinylbutyrals for
glass laminates, acrylics, and polystyrene as protective covers and
substrates, as well as cellulosic esters, polycarbonates, and
polyarylates, polyolefins, fluoroplastics (e.g. polyvinylfluoride
and polyvinylidene fluoride), sulfones for protective covers,
compensation plates, and electrode substrates. In the method of the
present invention, there are no particular limitations as to the
type polymers or blends of polymers that may be used to form
optical films.
[0061] In terms of solvents for the aforementioned resin materials,
suitable solvents include, for example, chlorinated solvents
(methylene chloride and 1,2 dichloroethane), alcohols (methanol,
ethanol, n-propanol, isopropanol, n-butanol, isobutanol, diacetone
alcohol, phenol, and cyclohexanol), ketones (acetone, methylethyl
ketone, methylisobutyl ketone, and cyclohexanone), esters (methyl
acetate, ethyl acetate, n-propyl acetate, isopropyl acetate,
isobutyl acetate, and n-butyl acetate), aromatics (toluene and
xylenes) and ethers (tetrahydrofuran, 1,3-dioxolane, 1,2-dioxolane,
1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). Water may also be used
as a solvent. Coating solutions may also be prepared with a blend
of the aforementioned solvents.
[0062] Coating fluids may also contain additives to act as
converting aids. Converting aids include plasticizers and
surfactants, and these additives are generally specific to the type
of polymer film. For example, plasticizers suitable for
polycarbonate, polyethersulfone, and cellulose triacetate films
include phthalate esters (diethylphthalate, dibutylphthalate,
dicyclohexylphthalate, dioctylphthalate, and butyl octylphthalate),
adipate esters (dioctyl adipate), and phosphate esters (tricresyl
phosphate and triphenyl phosphate). For the water-soluble polyvinyl
alcohols, on the other hand, suitable plasticizers include
polyhydric alcohols such as glycerin and ethylene glycol as well as
amine alcohols such as ethanolamine. Plasticizers may be used here
as coating aids in the converting operation to minimize premature
film solidification at the coating hopper and to improve drying
characteristics of the wet film. In the method of the present
invention, plasticizers may be used to minimize blistering, curl
and delamination of resin films during the drying operation. In a
preferred embodiment of the present invention, plasticizers may be
added to the coating fluid at a total concentration of up to 50% by
weight relative to the concentration of polymer in order to
mitigate defects in the final resin film.
[0063] The coating formulation for the low birefringence polymer
may also contain one or more UV absorbing compounds to provide UV
filter element performance and/or act as UV stabilizers for the low
birefringence polymer film. Ultraviolet absorbing compounds are
generally contained in the polymer in an amount of 0.01 to 20
weight parts based on 100 weight parts of the polymer containing no
ultraviolet absorber, and preferably contained in an amount of 0.01
to 10 weight parts, especially in an amount of 0.05 to 2 weight
parts. Any of the various ultraviolet light absorbing compounds
which have been described for use in various polymeric elements may
be employed in the polymeric elements of the invention, such as
hydroxyphenyl-s-triazine, hydroxyphenylbenzotriazole, formamidine,
or benzophenone compounds. As described in copending, commonly
assigned U.S. patent application U.S. Ser. No. 10/150,634, filed
May 5, 2002, the use of dibenzoylmethane ultraviolet absorbing
compounds in combination with a second UV absorbing compound such
as those listed above have been found to be particularly
advantageous with respect to providing both a sharp cut off in
absorption between the UV and visible light spectral regions as
well as increased protection across more of the UV spectrum.
Additional possible UV absorbers which may be employed include
salicylate compounds such as 4-t-butylphenylsalicylate; and
[2,2'thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Most
preferred are combinations of dibenzoylmethane compounds with
hydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole
compounds.
[0064] Dibenzoylmethane compounds which may be employed include
those of the formula (IV) ##STR2## where R1 through R5 are each
independently hydrogen, halogen, nitro, or hydroyxl, or further
substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy,
ester, carboxyl, alkyl thio, aryl thio, alkyl amine, aryl amine,
alkyl nitrile, aryl nitrile, arylsulfonyl, or 5-6 member
heterocylce ring groups. Preferably, each of such groups comprises
20 or fewer carbon atoms. Further preferably, R1 through R5 of
Formula IV are positioned in accordance with Formula IV-A: ##STR3##
Particularly preferred are compounds of Formula IV-A where R1 and
R5 represent alkyl or alkoxy groups of from 1-6 carbon atoms and R2
through R4 represent hydrogen atoms.
[0065] Representative compounds of Formula (IV) which may be
employed in accordance the elements of the invention include the
following: (IV-1): 4-(1,1-dimethylethyl)-4'-methoxydibenzoylmethane
(PARSOL.RTM. 1789) (IV-2): 4-isopropyl dibenzoylmethane
(EUSOLEX.RTM. 8020) (IV-3): dibenzoylmethane (RHODIASTAB.RTM.
83)
[0066] Hydroxyphenyl-s-triazine compounds which may be used in the
elements of the invention, e.g., may be a derivative of
tris-aryl-s-triazine compounds as described in U.S. Pat. No.
4,619,956. Such compounds may be represented by Formula V: ##STR4##
wherein X, Y and Z are each aromatic, carbocylic radicals of less
than three 6-membered rings, and at least one of X, Y and Z is
substituted by a hydroxy group ortho to the point of attachment to
the triazine ring; and each of R1 through R9 is selected from the
group consisting of hydrogen, hydroxy, alkyl, alkoxy, sulfonic,
carboxy, halo, haloalkyl and acylamino. Particularly preferred are
hydroxyphenyl-s-triazines of the formula V-A: ##STR5## wherein R is
hydrogen or alkyl of 1-18 carbon atoms.
[0067] Hydroxyphenylbenzotriazole compounds which may be used in
the elements of the invention, e.g., may be a derivative of
compounds represented by Formula VI: ##STR6## wherein R1 through R5
may be independently hydrogen, halogen, nitro, hydroxy, or further
substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy,
aryloxy, alkylthio, mono or dialkyl amino, acyl amino, or
heterocyclic groups. Specific examples of benzotriazole compounds
which may be used in accordance with the invention include
2-(2'-hydroxy-3'-t-butyl-5'-methylphenyl)-5-chlorobenzotriazole;
2-(2'-hydroxy-3',5'-di-t-amylphenyl)benzotriazole; octyl
5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzenepropionat-
e; 2-(hydroxy-5-t-octylphenyl)benzotriazole;
2-(2'-hydroxy-5'-methylphenyl)benzotriazole;
2-(2'-hydroxy-3'-dodecyl-5'-methylphenyl)benzotriazole; and
2-(2'-hydroxy-3',5'-di-t-butylphenyl)-5-chlorobenzotriazole.
[0068] Formamidine compounds which may be used in the elements of
the invention, e.g., may be a formamidine compound as described in
U.S. Pat. No. 4,839,405. Such compounds may be represented by
Formula VII or Formula VIII: ##STR7## wherein R1 is an alkyl group
containing 1 to about 5 carbon atoms; Y is a H, OH, Cl or an alkoxy
group; R2 is a phenyl group or an alkyl group containing 1 to about
9 carbon atoms; X is selected from the group consisting of H,
carboalkoxy, alkoxy, alkyl, dialkylamino and halogen; and Z is
selected from the group consisting of H, alkoxy and halogen;
##STR8## wherein A is --COOR, --COOH, --CONR'R'', --NR'COR, --CN,
or a phenyl group; and wherein R is an alkyl group of from 1 to
about 8 carbon atoms; R' and R'' are each independently hydrogen or
lower alkyl groups of from 1 to about 4 carbon atoms. Specific
examples of formamidine compounds which may be used in accordance
with the invention include those described in U.S. Pat. No.
4,839,405, and specifically
4-[[(methylphenylamino)methylene]amino]-ethyl ester.
[0069] Benzophenone compounds which may be used in the elements of
the invention, e.g., may include
2,2'-dihydroxy-4,4'dimethoxybenzophenone,
2-hydroxy-4-methoxybenzophenone and
2-hydroxy-4-n-dodecyloxybenzophenone.
[0070] Coating fluids may also contain surfactants as coating aids
to control artifacts related to flow after coating. Artifacts
created by flow after coating phenomena include mottle,
repellencies, orange-peel (Bernard cells), and edge-withdraw. For
polymeric resins dissolved in organic solvents, surfactants used
control flow after coating artifacts include siloxane and
fluorochemical compounds. Examples of commercially available
surfactants of the siloxane type include: 1.) Polydimethylsiloxanes
such as DC200 Fluid from Dow Corning, 2.) Poly(dimethyl,
methylphenyl)siloxanes such as DC510 Fluid from Dow Corning, and
3.) Polyalkyl substituted polydimethysiloxanes such as DC190 and
DC1248 from Dow Corning as well as the L7000 Silwet series (L7000,
L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkyl
substituted poly(dimethyl, methylphenyl)siloxanes such as SF1023
from General Electric. Examples of commercially available
fluorochemical surfactants include: 1.) Fluorinated alkyl esters
such as the Fluorad series (FC430 and FC431) from the 3M
Corporation, 2.) Fluorinated polyoxyethylene ethers such as the
Zonyl series (FSN, FSN100, FSO, FSO100) from Du Pont, 3.) Acrylate
polyperfluoroalkyl ethylacrylates such as the F series (F270 and
F600) from NOF Corporation, and 4.) Perfluoroalkyl derivatives such
as the Surflon series (S383, S393, and S8405) from the Asahi Glass
Company.
[0071] For polymeric resins dissolved in aqueous solvents,
appropriate surfactants include those suitable for aqueous coating
as described in numerous publications (see for example Surfactants:
Static and dynamic surface tension by YM Tricot in Liquid Film
Coating, pp 99-136, S E Kistler and P M Schweitzer editors, Chapman
and Hall [1997]). Surfactants may include nonionic, anionic,
cationic and amphoteric types. Examples of practical surfactants
include polyoxyethylene ethers, such as polyoxyethylene (8)
isooctylphenyl ether, polyoxyethylene (10) isooctylphenyl ether,
and polyoxyethylene (40) isooctylphenyl ether, and fluorinated
polyoxyethylene ethers such as the Zonyl series commercially
available from Du Pont.
[0072] In the method of the present invention, there are no
particular limits as to the type of surfactant used. In the method
of the present invention, surfactants are generally of the
non-ionic type. In a preferred embodiment of the present invention,
non-ionic compounds of either the siloxane or fluorinated type are
added to the uppermost layers when films are prepared with organic
solvents.
[0073] In terms of surfactant distribution, surfactants are most
effective when present in the uppermost layers of the multilayer
coating. In the uppermost layer, the concentration of surfactant is
preferably 0.001-1.000% by weight and most preferably 0.010-0.500%.
In addition, lesser amounts of surfactant may be used in the second
uppermost layer to minimize diffusion of surfactant away from the
uppermost layer. The concentration of surfactant in the second
uppermost layer is preferably 0.000-0.200% by weight and most
preferably between 0.000-0.100% by weight. Because surfactants are
only necessary in the uppermost layers, the overall amount of
surfactant remaining in the final dried film is small.
[0074] Although surfactants are not required to practice the method
of the current invention, surfactants do improve the uniformity of
the coated film. In particular, mottle nonuniformities are reduced
by the use of surfactants. In transparent resin films, mottle
nonuniformities are not readily visualized during casual
inspection. To visualize mottle artifacts, organic dyes may be
added to the uppermost layer to add color to the coated film. For
these dyed films, nonuniformities are easy to see and quantify. In
this way, effective surfactant types and levels may be selected for
optimum film uniformity.
[0075] Turning next to FIGS. 4 through 7, there are presented
cross-sectional illustrations showing various film configurations
prepared by the method of the present invention. In FIG. 4, a
single-layer optical film 150 is shown partially peeled from a
carrier substrate 152 that has been modified to possess a surface
energy less than 35 erg/cm.sup.2. Optical film 150 may be formed
either by applying a single liquid layer to the carrier substrate
152 or by applying a multiple layer composite having a composition
that is substantially the same among the layers. Alternatively in
FIG. 5, the carrier substrate 154 may have been pretreated with a
coated surface layer 156 that modifies the surface energy of the
substrate to less than 35 erg/cm.sup.2. FIG. 6 illustrates a
multiple layer film 160 that is comprised of four compositionally
discrete layers including a lowermost layer 162 nearest to the
carrier support 170 that has been modified to possess a surface
energy less than 35 erg/cm.sup.2, two intermediate layers 164, 166,
and an uppermost layer 168. FIG. 6 also shows that the entire
multiple layer composite 160 may be peeled from the carrier
substrate 170. FIG. 7 shows a multiple layer composite film 172
comprising a lowermost layer 174 nearest to the carrier substrate
182, two intermediate layers 176, 178, and an uppermost layer 180
being peeled from the carrier substrate 182. The carrier substrate
182 has been treated with a coated surface layer 184 to modify the
surface energy of the substrate 182. Coated surface layers 156 and
184 may be comprised of any polymeric materials that provide a
surface energy of less than 35 erg/cm.sup.2. Exemplary examples of
theses include: fluorinated and chlorinated polymers and latexes,
e.g., poly(tetrafluoroethylene), poly(hexafluoropropylene),
poly(trifluoroethylene), vinylidene fluoride interpolymers,
vinylidene chloride interpolymers, and
poly(trifluorochloroethylethylene; polystyrene; polyolefins, e.g.,
polyethylene and polypropylene; silicone polymers; and others. The
coated surface layers may be applied by aqueous or organic solvent
coating methods, by melt extrusion coating methods, by vacuum
coating methods, or other well known surface coating methods. The
choice of materials used in the coated surface layer may be
optimized empirically by those skilled in the art to achieve the
desired surface energy.
[0076] The method of the present invention may also include the
step of coating over a previously prepared composite of resin film
and carrier substrate. For example, the coating and drying system
10 shown in FIGS. 1 and 2 may be used to apply a second multilayer
film to an existing optical film/substrate composite. If the
film/substrate composite is wound into rolls before applying the
subsequent coating, the process is called a multi-pass coating
operation. If coating and drying operations are carried out
sequentially on a machine with multiple coating stations and drying
ovens, then the process is called a tandem coating operation. In
this way, thick films may be prepared at high line speeds without
the problems associated with the removal of large amounts of
solvent from a very thick wet film. Moreover, the practice of
multi-pass or tandem coating also has the advantage of minimizing
other artifacts such as streak severity, mottle severity, and
overall film nonuniformity.
[0077] The practice of tandem coating or multi-pass coating
requires some minimal level of adhesion between the first-pass film
and the carrier substrate. In some cases, film/substrate composites
having poor adhesion are observed to blister after application of a
second or third wet coating in a multi-pass operation. To avoid
blister defects, adhesion must be greater than 0.3 N/m between the
first-pass resin film and the carrier substrate. This level of
adhesion may be attained by a variety of web treatments including
various subbing layers and various electronic discharge treatments.
However, excessive adhesion between the applied film and substrate
is undesirable since the film may be damaged during subsequent
peeling operations. In particular, film/substrate composites having
an adhesive force of greater than 250 N/m have been found to peel
poorly. Films peeled from such excessively, well-adhered composites
exhibit defects due to tearing of the film and/or due to cohesive
failure within the film. In a preferred embodiment of the present
invention, the adhesion between the resin film and the carrier
substrate is less than 250 N/m. Most preferably, the adhesion
between resin film and the carrier substrate is between 0.5 and 25
N/m.
[0078] The method of the present invention is suitable for
application of resin coatings to a variety of carrier substrates
such as polyethylene terephthalate (PET), polyethylene naphthalate
(PEN), polystyrene, cellulose triaceate and other polymeric films.
Polymeric substrates may be unstretched, unixially stretched or
biaxially stretched films or sheets. Additional substrates may
include paper, laminates of paper and polymeric films, glass,
cloth, aluminum and other metal supports. In some cases, substrates
may be pretreated with subbing layers or electrical discharge
devices. Substrates may also be pretreated with functional layers
containing various binders and addenda. There are no particular
requirements regarding the thickness of the substrate. For the
optical resin films prepared here the substrate is PET with a
thickness of either 100 or 175 .mu.m. The method of the present
invention may be practiced using substrates having a thickness of 5
to 500 .mu.m.
[0079] The prior art method of casting resin films is illustrated
in FIG. 8. As shown in FIG. 8, a viscous polymeric dope is
delivered through a feed line 200 to an extrusion hopper 202 from a
pressurized tank 204 by a pump 206. The dope is cast onto a highly
polished metal drum 208 located within a first drying section 210
of the drying oven 212. The cast film 214 is allowed to partially
dry on the moving drum 208 and is then peeled from the drum 208.
The cast film 214 is then conveyed to a final drying section 216 to
remove the remaining solvent. The final dried film 218 is then
wound into rolls at a wind-up station 220. The prior art cast film
typically has a thickness in the range of from 40 to 200 .mu.m.
[0080] Coating methods are distinguished from casting methods by
the process steps necessary for each technology. These process
steps in turn affect a number of tangibles such as fluid viscosity,
converting aids, substrates, and hardware that are unique to each
method. In general, coating methods involve application of dilute
low viscosity liquids to thin flexible substrates, evaporating the
solvent in a drying oven, and winding the dried film/substrate
composite into rolls. In contrast, casting methods involve applying
a concentrated viscous dope to a highly polished metal drum or
band, partially drying the wet film on the metal substrate,
stripping the partially dried film from the substrate, removing
additional solvent from the partially dried film in a drying oven,
and winding the dried film into rolls. In terms of viscosity,
coating methods require very low viscosity liquids of less than
5,000 cp. In the practice of the method of the present invention
the viscosity of the coated liquids will generally be less than
2000 cp and most often less than 1500 cp. Moreover, in the method
of the present invention the viscosity of the lowermost layer is
preferred to be less than 200 cp. and most preferably less than 100
cp. for high speed coating application. In contrast, casting
methods require highly concentrated dopes with viscosity on the
order of 10,000-100,000 cp for practical operating speeds. In terms
of converting aids, coating methods generally involve the use of
surfactants as converting aids to control flow after coating
artifacts such as mottle, repellencies, orange peel, and edge
withdraw. In contrast, casting methods do not require surfactants.
Instead, converting aids are only used to assist in the stripping
and conveyance operations in casting methods. For example, lower
alcohols are sometimes used as converting aids in cast optical
films to minimize abrasion artifacts during conveyance through
drying ovens. In terms of substrates, coating methods generally
utilize thin (10-250 micron) flexible supports. In contrast,
casting methods employ thick (1-100 mm), continuous, highly
polished metal drums or rigid bands. As a result of these
differences in process steps, the hardware used in coating is
conspicuously different from those used in casting as can be seen
by a comparison of the schematics shown in FIGS. 1 and 7,
respectively.
[0081] FIG. 9 shows the dependence of the out-of-plane retardation
on film thickness for a polycarbonate resin film cast from a
methylene chloride solution onto a variety of substrates with
different surface energies. In-plane retardation for these
polycarbonate films were all essentially equal to zero. Retardation
and surface energy were determined as outlined below.
[0082] Retardation. Retardation of resin films were determined in
nanometers (nm) using a Woollam M-2000V Spectroscopic Ellipsometer
at wavelengths from 370 to 1000 nm. In-plane and out-of-plane
retardation are defined by the formula:
R.sub.e=|n.sub.x-n.sub.y|.times.d
R.sub.OOP=|[0.5(n.sub.x+n.sub.y)-n.sub.z}]|.times.d where R.sub.e
is the in-plane retardation at 590 nm, R.sub.OOP (or OPR) is the
out-of-plane retardation at 590 nm, n.sub.x is the index of
refraction of the peeled film along the machine direction, n.sub.y
is the is the index of refraction of the peeled film along the
transverse direction, n.sub.z is the refractive index of the film
parallel to its plane and d is the thickness of the peeled film in
nanometers (nm). Thus, R.sub.e is the absolute value of the
difference in refractive index between the machine and transverse
directions of the peeled film multiplied by the thickness of the
film. R.sub.oop is similarly the difference between the average of
the refractive indices in the machine and transverse directions and
the refractive index parallel to the plane of the film multiplied
by the the thickness of the film.
[0083] Surface Energy. The surface energy of a solid surface is
determined using the Girifalco-Good-Fowkes equation. This equation
requires the measurement of contact angles between the solid
surface and a drop of liquid and the surface tension, dispersive
force and polar force of the liquid. The measurements for this
invention were made using distilled water (a polar liquid) and
methylene iodide (a nonpolar liquid). The surface tension,
dispersive forces and polar forces of the two liquids are listed in
the table below TABLE-US-00001 Surface Tension Dispersive Force
Polar Force Liquid (erg/cm.sup.2) (erg/cm.sup.2) (erg/cm.sup.2)
Water 72.8 21.8 51 Methylene iodide 50.8 48.5 2.3
The contact angle measurements were performed on clean surfaces at
room temperature, 20 C, using a Rame-Hart Goniometer with a white
light source. The surfaces were cleaned by repeated rinses with
acetone, isopropanol and distilled water. A drop of distilled water
or methylene iodide was placed on the clean surface and the contact
angle was measured on each side of the drop, for 2-3 drops for each
liquid.
[0084] In accordance with the invention, composite elements may be
prepared that comprise a carrier substrate having a low surface
energy, an optical resin film having an out-of-plane retardation of
less than 100 nm and an in-plane retardation of less than 20 nm,
and one or more auxiliary layers that are applied on the said
optical resin film. The one or more auxiliary layers may be applied
simultaneously with the optical resin film or they may be applied
in a separate coating operation. Suitable auxiliary layers for use
in the present invention include PVA adhesion-promoting layer,
abrasion resistant hardcoat layer, antiglare layer, anti-smudge
layer or stain-resistant layer, antireflection layer, low
reflection layer, antistatic layer, viewing angle compensation
layer, and moisture barrier layer. Optionally, the composite
element of the invention may also comprise a strippable, protection
layer on the side of the composite element opposite to the carrier
substrate.
[0085] Particularly effective abrasion resistant layers for use in
the present invention comprise radiation or thermally cured
compositions, and preferably the composition is radiation cured.
Ultraviolet (UV) radiation and electron beam radiation are the most
commonly employed radiation curing methods. UV curable compositions
are particularly useful for creating the abrasion resistant layer
of this invention and may be cured using two major types of curing
chemistries, free radical chemistry and cationic chemistry.
Acrylate monomers (reactive diluents) and oligomers (reactive
resins and lacquers) are the primary components of the free radical
based formulations, giving the cured coating most of its physical
characteristics. Photo-initiators are required to absorb the UV
light energy, decompose to form free radicals, and attack the
acrylate group C.dbd.C double bond to initiate polymerization.
Cationic chemistry utilizes cycloaliphatic epoxy resins and vinyl
ether monomers as the primary components. Photo-initiators absorb
the UV light to form a Lewis acid, which attacks the epoxy ring
initiating polymerization. By UV curing is meant ultraviolet curing
and involves the use of UV radiation of wavelengths between 280 and
420 nm preferably between 320 and 410 nm.
[0086] Examples of UV radiation curable resins and lacquers usable
for the abrasion layer useful in this invention are those derived
from photo polymerizable monomers and oligomers such as acrylate
and methacrylate oligomers (the term "(meth)acrylate" used herein
refers to acrylate and methacrylate), of polyfunctional compounds,
such as polyhydric alcohols and their derivatives having
(meth)acrylate functional groups such as ethoxylated
trimethylolpropane tri(meth)acrylate, tripropylene glycol
di(meth)acrylate, trimethylolpropane tri(meth)acrylate, diethylene
glycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate,
pentaerythritol tri(meth)acrylate, dipentaerythritol
hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl
glycol di(meth)acrylate and mixtures thereof, and acrylate and
methacrylate oligomers derived from low-molecular weight polyester
resin, polyether resin, epoxy resin, polyurethane resin, alkyd
resin, spiroacetal resin, epoxy acrylates, polybutadiene resin, and
polythiol-polyene resin, and the like and mixtures thereof, and
ionizing radiation-curable resins containing a relatively large
amount of a reactive diluent. Reactive diluents usable herein
include monofunctional monomers, such as ethyl (meth)acrylate,
ethylhexyl (meth)acrylate, styrene, vinyltoluene, and
N-vinylpyrrolidone, and polyfunctional monomers, for example,
trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate,
tripropylene glycol di(meth)acrylate, diethylene glycol
di(meth)acrylate, pentaerythritol tri(meth)acrylate,
dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol
di(meth)acrylate, or neopentyl glycol di(meth)acrylate.
[0087] An example of a UV radiation curable resin that is
conveniently used in the practice of this invention is CN 968.RTM.
from Sartomer Company. The abrasion resistant layer of the
invention typically provides a layer having a pencil hardness
(using the Standard Test Method for Hardness by Pencil Test ASTM
D3363) of at least 2H and preferably 2H to 8H.
[0088] The composite element of the invention may contain an
antiglare layer, a low reflection layer or an antireflection layer.
Such layers are employed in an LCD in order to improve the viewing
characteristics of the display, particularly when it is viewed in
bright ambient light.
[0089] An antiglare coating provides a roughened or textured
surface that is used to reduce specular reflection. All of the
unwanted reflected light is still present, but it is scattered
rather than specularly reflected. For the purpose of the present
invention, the antiglare coating preferably comprises a radiation
cured composition that has a textured or roughened surface obtained
by the addition of organic or inorganic (matting) particles or by
embossing the surface. The radiation cured compositions described
hereinabove for the abrasion resistant layer are also effectively
employed in the antiglare layer. Surface roughness is preferably
obtained by the addition of matting particles to the radiation
cured composition. Suitable particles include inorganic compounds
having an oxide, nitride, sulfide or halide of a metal, metal
oxides being particularly preferred. As the metal atom, Na, K, Mg,
Ca, Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta,
Ag, Si, B, Bi, Mo, Ce, Cd, Be, Pb and Ni are suitable, and Mg, Ca,
B and Si are more preferable. An inorganic compound containing two
types of metal may also be used. A particularly preferable
inorganic compound is silicon dioxide, namely silica.
[0090] Additional particles suitable for use in the antiglare layer
of the present invention include the layered clays described in
commonly-assigned U.S. patent application Ser. No. 10/690,123,
filed Oct. 21, 2003. The most suitable layered particles include
materials in the shape of plates with high aspect ratio, which is
the ratio of a long direction to a short direction in an asymmetric
particle. Preferred layered particles are natural clays, especially
natural smectite clay such as montmorillonite, nontronite,
beidellite, volkonskoite, hectorite, saponite, sauconite,
sobockite, stevensite, svinfordite, halloysite, magadiite, kenyaite
and vermiculite as well as layered double hydroxides or
hydrotalcites. Most preferred clay materials include natural
montmorillonite, hectorite and hydrotalcites, because of commercial
availability of these materials.
[0091] Additional particles for use in the antiglare layer of the
present invention include polymer matte particles or beads which
are well known in the art. The polymer particles may be solid or
porous, preferably they are crosslinked polymer particles. Porous
polymer particles for use in an antiglare layer are described in
commonly-assigned U.S. patent application Ser. No. 10/715,706,
filed Nov. 18, 2003.
[0092] Particles for use in the antiglare layer have an average
particle size ranging from 2 to 20 micrometers, preferably from 2
to 15 micrometers and most preferably from 4 to 10 micrometers.
They are present in the layer in an amount of at least 2 wt percent
and less than 50 percent, typically from about 2 to 40 wt. percent,
preferably from 2 to 20 percent and most preferably from 2 to 10
percent.
[0093] The thickness of the antiglare layer is generally about 0.5
to 50 micrometers preferably 1 to 20 micrometers more preferably 2
to 10 micrometers.
[0094] Preferably, the antiglare layer used in the present
invention has a 60.degree. Gloss value, according to ASTM D523, of
less than 100, preferably less than 90 and a transmission haze
value, according to ASTM D-1003 and JIS K-7105 methods, of less
than 50%, preferably less than 30%.
[0095] In another embodiment of the present invention, a low
reflection layer or antireflection layer is used in combination
with an abrasion resistant hard coat layer or antiglare layer. The
low reflection or antireflection coating is applied on top of the
abrasion resistant or antiglare layer. Typically, a low reflection
layer provides an average specular reflectance (as measured by a
spectrophotometer and averaged over the wavelength range of 450 to
650 nm) of less than 2%. Antireflection layers provide average
specular reflectance values of less than 1%.
[0096] Suitable low reflection layers for use in the present
invention comprise fluorine-containing homopolymers or copolymers
having a refractive index of less than 1.48, preferably with a
refractive index between about 1.35 and 1.40. Suitable
fluorine-containing homopolymers and copolymers include:
fluoro-olefins (for example, fluoroethylene, vinylidene fluoride,
tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene,
perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely
fluorinated alkyl ester derivatives of (meth)acrylic acid, and
completely or partially fluorinated vinyl ethers, and the like. The
effectiveness of the layer may be improved by the incorporation of
submicron-sized inorganic particles or polymer particles that
induce interstitial air voids within the coating. This technique is
further described in U.S. Pat. No. 6,210,858 and U.S. Pat. No.
5,919,555. Further improvement of the effectiveness of the low
reflection layer may be realized with the restriction of air voids
to the internal particle space of submicron-sized polymer particles
with reduced coating haze penalty, as described in
commonly-assigned U.S. patent application Ser. No. 10/715,655,
filed Nov. 18, 2003.
[0097] The thickness of the low reflection layer is 0.01 to 1
micrometer and preferably 0.05 to 0.2 micrometer.
[0098] An antireflection layer may comprise a monolayer or a
multi-layer. Antireflection layers comprising a monolayer typically
provide reflectance values less than 1% at only a single wavelength
(within the broader range of 450 to 650 nm). A commonly employed
monolayer antireflection coating that is suitable for use in the
present invention comprises a layer of a metal fluoride such as
magnesium fluoride (MgF.sub.2). The layer may be applied by
well-known vacuum deposition technique or by a sol-gel technique.
Typically, such a layer has an optical thickness (i.e., the product
of refractive index of the layer times layer thickness) of
approximately one quarter-wavelength at the wavelength where a
reflectance minimum is desired.
[0099] Although a monolayer can effectively reduce the reflection
of light within a very narrow wavelength range, more often a
multi-layer comprising several (typically, metal oxide based)
transparent layers superimposed on one another is used to reduce
reflection over a wide wavelength region (i.e., broadband
reflection control). For such a structure, half wavelength layers
are alternated with quarter wavelength layers to improve
performance. The multi-layer antireflection coating may comprise
two, three, four, or even more layers. Formation of this
multi-layer typically requires a complicated process comprising a
number of vapor deposition procedures or sol-gel coatings, which
correspond to the number of layers, each layer having a
predetermined refractive index and thickness. Precise control of
the thickness of each layer is required for these interference
layers. The design of suitable multi-layer antireflection coatings
for use in the present invention is well known in the patent art
and technical literature, as well as being described in various
textbooks, for example, in H. A. Macleod, "Thin Film Optical
Filters," Adam Hilger, Ltd., Bristol 1985 and James D. Rancourt,
"Optical Thin Films User's Handbook", Macmillan Publishing Company,
1987.
[0100] The composite elements of the invention may contain a
moisture barrier layer comprising a hydrophobic polymer such as a
vinylidene chloride polymer, vinylidene fluoride polymer,
polyurethane, polyolefin, fluorinated polyolefin, polycarbonate,
and others, having a low moisture permeability. Preferably, the
hydrophobic polymer comprises vinylidene chloride. More preferably,
the hydrophobic polymer comprises 70 to 99 weight percent of
vinylidene chloride. The moisture barrier layer may be applied by
application of an organic solvent-based or aqueous coating
formulation. To provide effective moisture barrier properties the
layer should be at least 1 micrometer in thickness, preferably from
1 to 10 micrometers in thickness, and most preferably from 2 to 8
micrometers in thickness. The moisture barrier layer has a moisture
vapor transmission rate (MVTR) according to ASTM F-1249 that is
less than 1000 g/m.sup.2/day, preferably less than 800
g/m.sup.2/day and most preferably less than 500 g/m.sup.2/day. The
use of such a barrier layer on the optical resin films of the
invention provides improved resistance of the optical film to
changes in humidity.
[0101] The composite elements of the invention may contain a
transparent antistatic layer that aids in the control of static
charging that may occur during the manufacture and use of the
composite element and optical film. The composite elements of the
invention may be particularly prone to triboelectric charging
during the peeling of the optical resin film from the carrier
substrate. The so-called "separation charge" that results from the
separation of the resin film and the substrate can be effectively
controlled by an antistatic layer having a resistivity of less than
about 1.times.10.sup.11 .OMEGA./square, preferably less than
1.times.10.sup.10 .OMEGA./square, and most preferably less than
1.times.10.sup.9 .OMEGA./square.
[0102] Conductive materials employed in the antistatic layer may be
either ionically-conductive or electronically-conductive.
Ionically-conductive materials include simple inorganic salts,
alkali metal salts of surfactants, polymeric electrolytes
containing alkali metal salts, and colloidal metal oxide sols
(stabilized by metal salts). Of these, ionically-conductive
polymers such as anionic alkali metal salts of styrene sulfonic
acid copolymers and cationic quaternary ammonium polymers of U.S.
Pat. No. 4,070,189 and ionically-conductive colloidal metal oxide
sols which include silica, tin oxide, titania, antimony oxide,
zirconium oxide, alumina-coated silica, alumina, boehmite, and
smectite clays are preferred.
[0103] The antistatic layer employed in the current invention
preferably contains an electronically-conductive material due to
their humidity and temperature independent conductivity. Suitable
materials include: [0104] 1) electronically-conductive
metal-containing particles including donor-doped metal oxides,
metal oxides containing oxygen deficiencies, and conductive
nitrides, carbides, and bromides. Specific examples of particularly
useful particles include conductive SnO.sub.2, In.sub.2O,
ZnSb.sub.2O.sub.6, InSbO.sub.4, TiB.sub.2, ZrB.sub.2, NbB.sub.2,
TaB.sub.2, CrB, MoB, WB, LaB.sub.6, ZrN, TiN, WC, HfC, HfN, and
ZrC. Examples of the patents describing these electrically
conductive particles include; U.S. Pat. Nos. 4,275,103; 4,394,441;
4,416,963; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276;
5,122,445; and 5,368,995. [0105] 2) fibrous electronic conductive
particles comprising, for example, antimony-doped tin oxide coated
onto non-conductive potassium titanate whiskers as described in
U.S. Pat. Nos. 4,845,369 and 5,166,666, antimony-doped tin oxide
fibers or whiskers as described in U.S. Pat. Nos. 5,719,016 and
5,0731,119, and the silver-doped vanadium pentoxide fibers
described in U.S. Pat. No. 4,203,769 [0106] 3)
electronically-conductive polyacetylenes, polythiophenes, and
polypyrroles, preferably the polyethylene dioxythiophene described
in U.S. Pat. No. 5,370,981 and commercially available from Bayer
Corp. as Baytron.RTM. P.
[0107] The amount of the conductive agent used in the antistatic
layer of the invention can vary widely depending on the conductive
agent employed. For example, useful amounts range from about 0.5
mg/M.sup.2 to about 1000 mg/M.sup.2, preferably from about 1
mg/m.sup.2 to about 500 mg/m.sup.2. The antistatic layer has a
thickness of from 0.05 to 5 micrometers, preferably from 0.1 to 0.5
micrometers to insure high transparency.
[0108] Contrast, color reproduction, and stable gray scale
intensities are important quality attributes for electronic
displays, which employ liquid crystal technology. The primary
factor limiting the contrast of a liquid crystal display is the
propensity for light to "leak" through liquid crystal elements or
cells, which are in the dark or "black" pixel state. Furthermore,
the leakage and hence contrast of a liquid crystal display are also
dependent on the direction from which the display screen is viewed.
Typically the optimum contrast is observed only within a narrow
viewing angle range centered about the normal incidence to the
display and falls off rapidly as the viewing direction deviates
from the display normal. In color displays, the leakage problem not
only degrades the contrast but also causes color or hue shifts with
an associated degradation of color reproduction.
[0109] Thus, one of the major factors measuring the quality of LCDs
is the viewing angle characteristic, which describes a change in
contrast ratio from different viewing angles. It is desirable to be
able to see the same image from a wide variation in viewing angles
and this ability has been a shortcoming with liquid crystal display
devices. One way to improve the viewing angle characteristic is to
employ an optical resin film having a viewing angle compensation
layer (also referred to as a compensation layer, retarder layer, or
phase difference layer), with proper optical properties, between
the PVA-dichroic film and liquid crystal cell, such as disclosed in
U.S. Pat. Nos. 5,583,679, 5,853,801, 5,619,352, 5,978,055, and
6,160,597. A compensation film according to U.S. Pat. Nos.
5,583,679 and 5,853,801 based on discotic liquid crystals which
have negative birefringence, is widely used.
[0110] Viewing angle compensation layers useful in the present
invention are optically anisotropic layers. The optically
anisotropic, viewing angle compensation layers may comprise
positively birefringent materials or negatively birefringent
materials. The compensation layer may be optically uniaxial or
optically biaxial. The compensation layer may have its optic axis
tilted in the plane perpendicular to the layer. The tilt of the
optic axis may be constant in the layer thickness direction or the
tilt of the optic axis may vary in the layer thickness
direction.
[0111] Optically anisotropic, viewing angle compensation layers
useful in the present invention may comprise the negatively
birefringent, discotic liquid crystals described in U.S. Pat. Nos.
5,583,679, and 5,853,801; the positively birefringent nematic
liquid crystals described in U.S. Pat. No. 6,160,597; and the
negatively birefringent amorphous polymers described in
commonly-assigned U.S. patent application Publication No.
2004/0021814A and U.S. patent application Ser. No. 10/745,109,
filed Dec. 23, 2003.
[0112] The auxiliary layers of the invention may also include the
PVA adhesion-promoting layers described in commonly-assigned U.S.
patent application Ser. No. 10/838,841 filed May 4, 2004.
[0113] The auxiliary layers of the invention can be applied by any
of a number of well known liquid coating techniques, such as dip
coating, rod coating, blade coating, air knife coating, gravure
coating, microgravure coating, reverse roll coating, slot coating,
extrusion coating, slide coating, curtain coating, or by vacuum
deposition techniques. In the case of liquid coating, the wet layer
is generally dried by simple evaporation, which may be accelerated
by known techniques such as convection heating. The auxiliary layer
may be applied simultaneously with the optical resin film or it may
be applied after coating and drying of the optical resin film.
Several different auxiliary layers may be coated simultaneously
using slide coating, for example, an antistatic layer may be coated
simultaneously with a moisture barrier layer or a moisture barrier
layer may be coated simultaneously with a viewing angle
compensation layer. Known coating and drying methods are described
in further detail in Research Disclosure 308119, Published December
1989, pages 1007 to 1008.
[0114] The optical films of the invention are suitable for use with
a wide variety of LCD display modes, for example, Twisted Nematic
(TN), Super Twisted Nematic (STN), Optically Compensated Bend
(OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid
crystal displays. These various liquid crystal display technologies
have been reviewed in U.S. Pat. No. 5,619,352 (Koch et al.), U.S.
Pat. No. 5,410,422 (Bos), and U.S. Pat. No. 4,701,028 (Clerc et
al.).
[0115] As should be obvious based on the preceding detailed
description, a wide variety of composite elements and optical films
having various types and arrangements of auxiliary layers may be
prepared. Some of the configurations possible in accordance with
the present invention are illustrated by the following non-limiting
examples.
[0116] Composite C1: TABLE-US-00002 TAC polyethylene PET
[0117] A 100 micrometer thick polyethylene terephthalate (PET)
substrate is corona treated and extrusion coated with a 25
micrometer thick polyethylene layer (polyethylene has a surface
tension of 31 erg/cm.sup.2 based on results reported in "Adhesion
and Cohesion", Philip Weiss ed., p 190, Elsevier Publishing
Company, Amsterdam, 1962). A triacetyl cellulose (TAC) coating
formulation is applied from methylene chloride solution onto the
polyethylene surface. The dried TAC film is 20 micrometers in
thickness and contains 11 wt % triphenyl phosphate plasticizer, 1
wt % TINUVIN.RTM. 8515 UV absorber (a mixture of
2-(2'-Hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chloro benzotriazole
and 2-(2'-Hydroxy-3',5'-ditert-butylphenyl)-benzotriazole,
available from Ciba Specialty Chemicals.) and 0.1 wt % PARSOL.RTM.
1789 UV absorber (4-(1,1-dimethylethyl)-4'-methoxydibenzoylmethane,
available from Roche Vitamins Inc.).
[0118] Composite C2: TABLE-US-00003 abrasion resistant layer TAC
polyethylene PET
[0119] Composite C2 is prepared in an analogous manner to composite
C1 except an abrasion resistant layer, prepared by coating, drying
and then UV curing a urethane acrylate oligomer, CN 968.RTM. from
Sartomer Company, is applied onto the dried TAC film.
[0120] Composite C3: TABLE-US-00004 low reflection layer abrasion
resistant layer TAC polyethylene PET
[0121] Composite C3 is prepared in an analogous manner to composite
C2 except a low reflection layer having a thickness of 0.1
micrometers, comprising a fluorinated olefin polymer is applied
onto the abrasion resistant layer.
[0122] Composite C4: TABLE-US-00005 low reflection layer abrasion
resistant layer TAC polystyrene PET
[0123] Composite C4 is prepared in an analogous manner to composite
C3 except the polyethylene terephthalate substrate has aa 0.5
micrometer thick polystyrene layer rather than a polyethylene
layer. (Polystyrene has a surface tension of 33 erg/cm.sup.2 based
on results reported in "Adhesion and Cohesion", Philip Weiss ed., p
190, Elsevier Publishing Company, Amsterdam, 1962).
[0124] Composite C5: TABLE-US-00006 abrasion resistant layer
antistatic layer moisture barrier layer TAC polyethylene PET
[0125] Composite C5 is prepared in an analogous manner to composite
C1 except that a 5 micrometer thick moisture barrier layer
comprising poly(vinylidene chloride-co-acrylonitrile-co-acrylic
acid) containing 78 wt % vinylidene chloride is applied onto the
TAC layer. An antistatic layer comprising Baytron.RTM. P
(polyethylene dioxythiophene/polystyrene sulfonate, available from
Bayer Corp) in a poly(vinylidene
chloride-co-acrylonitrile-co-acrylic acid) binder is applied onto
the moisture barrier layer. The antistatic layer contains 3
mg/m.sup.2 Baytron.RTM. P and has a surface resistivity of about
1.times.10.sup.8 .OMEGA./square. The abrasion resistant layer
employed in composite C1 is applied onto the antistatic layer.
[0126] Composite C6: TABLE-US-00007 abrasion resistant layer
antistatic layer moisture barrier layer polycarbonate polyethylene
PET
[0127] Composite C6 is prepared in an analogous manner to composite
C5 except that a 20 micrometer thick layer of a polycarbonate,
Bisphenol A type homopolymer, is used in place of TAC as the low
birefringence polymer film.
[0128] Guarded cover sheet composite C7: TABLE-US-00008 TAC
polyethylene substrate
[0129] Guarded cover sheet composite C7 is prepared in an analogous
manner to composite C1 except that in place of a PET substrate
having a polyethylene extrusion coated layer, the carrier substrate
is a 100 micron thick polyethylene film.
[0130] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
[0131] From the foregoing, it will be seen that this invention is
one well adapted to obtain all of the ends and objects hereinabove
set forth together with other advantages which are apparent and
which are inherent to the apparatus.
[0132] It will be understood that certain features and
subcombinations are of utility and may be employed with reference
to other features and subcombinations. This is contemplated by and
is within the scope of the claims.
[0133] As many possible embodiments may be made of the invention
without departing from the scope thereof, it is to be understood
that all matter herein set forth and shown in the accompanying
drawings is to be interpreted as illustrative and not in a limiting
sense. The entire contents of the patents and other publications
referred to in this specification are incorporated herein by
reference.
Parts List
[0134] 10 drying system 66 drying section [0135] 12 moving
substrate/web 68 drying section [0136] 14 dryer 70 drying section
[0137] 16 coating apparatus 72 drying section [0138] 18 unwinding
station 74 drying section [0139] 20 back-up roller 76 drying
section [0140] 22 coated web 78 drying section [0141] 24 dry film
80 drying section [0142] 26 wind up station 82 drying section
[0143] 28 coating supply vessel 92 front section [0144] 30 coating
supply vessel 94 second section [0145] 32 coating supply vessel 96
third section [0146] 34 coating supply vessel 98 fourth section
[0147] 36 pumps 100 back plate [0148] 38 pumps 102 inlet [0149] 40
pumps 104 metering slot [0150] 42 pumps 106 pump [0151] 44 conduits
108 lower most layer [0152] 46 conduits 110 inlet [0153] 48
conduits 112 2.sup.nd metering slot [0154] 50 conduits 114 pump
[0155] 52 discharge device 116 layer [0156] 54 polar charge assist
device 118 inlet [0157] 56 opposing rollers 120 metering slot
[0158] 58 opposing rollers 122 pump [0159] 60 resin film 124 layer
[0160] 62 winding station 126 inlet [0161] 64 winding station 128
metering slot [0162] 130 pump 208 metal drum [0163] 132 layer 210
drying section [0164] 136 coating lip 212 drying oven [0165] 214
coating layer [0166] 138 2.sup.nd incline slide surface 216 final
drying section [0167] 140 3.sup.rd incline slide surface 218 final
dried film [0168] 142 4.sup.th incline slide surface 220 wind-up
station [0169] 144 back land surface [0170] 146 coating bead [0171]
150 resin film [0172] 152 carrier substrate [0173] 154 carrier
substrate [0174] 156 coated surface layer [0175] 158 resin film
[0176] 160 multiple layer film [0177] 162 lowermost layer [0178]
164 intermediate layers [0179] 166 intermediate layers [0180] 168
upper most layer [0181] 170 carrier support [0182] 172 composite
film [0183] 174 lower most layer [0184] 176 intermediate layers
[0185] 178 intermediate layers [0186] 180 upper most layers [0187]
182 carrier substrate [0188] 184 coated surface layer [0189] 200
feed line [0190] 202 extrusion hopper [0191] 204 pressurized tank
[0192] 206 pump
* * * * *